Method of producing layers of grains particularly monolayers of grains embedded in a filler

ABSTRACT

A method of manufacturing an electrical device in which a liquid electrically insulating filler substance is applied to a layer of electrically active grains which filler is allowed to contract thus exposing the peaks of the grains after which it is allowed to harden. The filler is preferably a resin hardenable by polymerization or by polycondensation in a liquid solution with a volatile solvent, for example polyurethane.

States Patent vent [ Mar, 114, 11972 154] METHUD @F PRQZDUTIHNG LAYERS 01F GRAINS YPARTHCULARZLY MUNUELAYERS @11 GRAHNS EMBEDDED [72] inventor: Ties Sielmlt Te Velde, Emmasingel, Eindhoven, Netherlands [73] Assignee: US. llrilips Corporation, New York, NY.

[22] Filed: Apr. 11, 1967 [21] Appl. No: 629,999

[30] Foreign Application Priority Data Apr. 14, 1966 Netherlands ..6604960 [52] 11.5. C1 ..117/217, 117/210, 117/212, 117/227,117/8,1l7/25,l17/26 {51] int. Cl ..C23b 5/50, C23b 5/64 [58] Field of Search ..117/210, 8, 25, 26, 212, 227; 264/102, 104, 129,155, 261; 96/36, 36.2, 38.2;

[56] Reterences Cited UNITED STATES PATENTS 2,201,196 5/1940 Williamson ..117/25 2,567,186 9/1951 Cross et al. ..117/26 2,904,613 9/1959 Paradise 136/89 3,031,344 4/1962 Sher et al..... 17/26 3,108,021 10/1963 Stanley v.117/8 3,291,578 12/1966 Fahey ..117/8 Primary ExaminerRalph S. Kendall Assistant ExaminerM. F. Esposito Att0rneyFrank R. Trifari [5 7 ABSTRACT A method of manufacturing an electrical device in which a liquid electrically insulating filler substance is applied to a layer of electrically active grains which filler is allowed to contract thus exposing the peaks of the grains after which it is allowed to harden. The filler is preferably a resin hardenable by polymerization or bypolycondensation in a liquid solution with a volatile solvent, for example polyurethane.

7 Claims, 11 Drawing Figures PATENTEDHARHHSYZ 3,649,354

SHEET 1 [1F 3 INVENTOR TIES 5 .TE VELDE M K-L/QS AGEN PATENTEU AR 14 19 2 3,649 35A SHEET 2 OF 3 FIG? VENT OR.

IN TIES S .TE VELDE BY jzwa K344i AGEN PATENTEDHAR 14 m2 3, 649 354 SHEET 3 OF 3 INVENTOR. TIES S .TE VELDE 71%. AGEN METHOD OF iRODlUCiNG LAYERS F GRAHNS PARTHCULARLY MONOLAYERS OF GRAHNS EMBEDDED TN A iFilLLER The invention relates to a method of producing a layer of grains embedded in a filler, preferably a layer one grain thick, termed hereinafter monolayer of grains, particularly for manufacturing electrically operative devices such as electrode systems, for example, a semiconductor electrode system in which the grains and the tiller are applied to the support, while at least on the side of the layer remote from the support sur face parts of the grains are made free of the tiller.

The invention furthermore relates to a layer of grains obtained by said method, particularly for use in an electrically operating device, for example, a semiconductor device. The layers of grains of the kind set forth may be employed for many purposes. Such layers of grains are particularly important for use in electrically active devices, for example, electrode systems in which a layer of grains of a medium electrically active in such a device in an insulating filler is employed. it is known to construct in this way capacitors comprising one or more layers of grains of ceramic material having a high dielectric constant and having a size substantially corresponding to the thickness of the layer, said grains being embedded in an insulating filler. The two electrodes of the capacitor are provided each on one side of such a layer or of a number of stacked layers. The grains may have the shape of scales which may be provided previously, if desired, with a metal coating on either side. The layer may be formed directly on a metal foil as a substrate. A difficulty involved is that a direct contact between the metal electrodes and the grains should be obtained as far as possible without the interposition of the tiller. Since the dielectric constant of the tiller is usually low with respect to that of the ceramic material, even a very thin film of filler between the grains and the electrode may markedly reduce the capacitance per unit of surface.

in my prior, copending patent applications Ser. Nos. 569,204 (now US. Pat. No. 3,480,818); 569,170 now abandoned; and 569,248 now abandoned, filed Aug. 1, 1966 it is described that layers of grains, preferably layers one grain thick comprising grains of an active medium in a filler may be used for other purposes than a capacitor, for example, in radiation detectors in which radiation energy is incident to a photosensitive layer in which it produces electric voltage differences or impedance variations, which are derived therefrom by means of the electrodes applied to the layer. There is furthermore described the possibility of use in the conversion of radiation energy into electric energy, inter alia in so-called solar batteries, and in the conversion of electric energy into radiation energy, for example by recombination radiation at PNjunctions in semiconductors and by other forms than electroluminescence. in most of these electrode systems a satisfactory, direct contact between the grains and the electrode applied to the layer is even essential. The present invention has for its object inter alia to provide a method which ensures that at least on the side of the layer remote from the support surface portions of the grains are free of the filler. The invention is based on the fact that a very great number of materials suitable for use as fillers can be brought into the liquid state, in which they tend to contract. According to the invention the filler is first used in the liquid state, so that the liquid tiller applied to the support contracts between the grains on the support and surface portions of the grains are free of the tiller on the side of the layer remote from the support, after which the tiller is hardened.

The method according to the invention may be carried out by means of a great variety of materials as a filler, which may be employed in the liquid state in various ways. There may be used, for example, fusible substances such as paraffin and solid esters of saturated fatty acids or thermoplastic substances, for example on the basis of polyvinyl resins an polyamides such as nylon. Moreover thermohardening and basically also cold-hardening materials, if desired, mixed with inhibitors and/or catalysts are appropriate substances, which are hardened from the liquid state by polymerization or polycondensation. Also materials hardening under the action of gaseous reactants may be chosen, for example, so-called drying oils and other liquid resins hardening under the action of oxygen. However, this range of optional fillers is not limited to the substances mentioned above. In principle, it is possible for example to use a solution of the filler, for example, in evaporable solvent.

in many of the aforesaid cases the change from the liquid state to the hardened state may be attended with a volume variation of the filler, particularly a reduction of the volume. Taking such a change of volume in consideration, the quantity offiller used in the liquid state has to be chosen so that during the contraction between the grains, the heads of the grains are sufficiently liberated, while after hardening of the tiller the layer of grains forms a porefree assembly, while the tiller itself anchors the grains because it embraces the grains sufficiently as far as above the center. There may be used a filler which adheres, in the liquid state, only sparingly to the surface of the grains concerned. For example, organic fillers of apolar nature or a predominantly apolar nature have, in general, poor adhesion to surfaces of grains or inorganic materials. in the liquid state these substances tend to form a more or less convex meniscus between the grains during the contraction. Fusible substances of this type are, for example, paraffin and solid esters of saturated fatty acids, such as fats and nondefonning kinds of wax. It will be appreciated that the quantity of filler may be comparatively large, while, if a satisfactory cohesion of the layer of grains by means of the tiller alone is desired the meniscus of the liquid between the grains may extend at least to some extent above the centers of the grains, it being even allowed for the filler to project locally above the heads of the grains.

As an alternative, however, a filler may be employed, which has such an adhesive force with respect to the surface of the grains, that in the liquid state the tiller tends to creep slightly up the surface of the grains. 1n the latter case the quantity of filler is preferably chosen so that the liquid level between the grains is lower than the level of the grain heads. In this case it is even permissible for the level of the filler between the grains to extend only to half the height of the grains or to a level below said half, whereas the filler covers nevertheless part of the grains surface above the center of the grain.

It is advantageous to apply the desired quantity of filler by using it in the form of a solution in or of a liquid mixture with a readily evaporable solvent. In order to obtain a layer of the thickness of a few grains, it is preferred to start with a support to which the grains are secured by means of a very thin adhesive layer, as is proposed in the aforesaid prior US. Pat. appli cation Ser. No. 569,248. The solution of the tiller may be caused to flow out on the support surface with the grains secured thereto, or the support with the grains may be dipped in the solution. The residual quantity of liquid depends upon the viscosity, which is usually lower according as the solution is more diluted.

The quantity of residual filler subsequent to the evaporation of the solvent depends furthermore upon the concentration of this filler in the solution. it will be obvious that by a correct choice of the degree of dilution of the tiller in the solution the quantity of filler in the layer can be controlled.

if the filler is solid prior to its solution, a liquid state will first be maintained, when the solvent is evaporated, until at the temperature of evaporation a state of saturation is reached or at least in which the tiller with the solvent is still capable of flowing. in principle, in this state the liquid can retract between the grains to an extent such that surface parts of the grains are set free, which also depends upon the quantity of liquid in this state. it should be taken into account that upon a further evaporation of the solvent an appreciable shrinkage may occur, which should certainly not be such that there is a risk of cracks or pores is involved or that the required cohesion or, as the case may be, the rigidity of the layer might get lost. in this case the filler is preferably brought to the liquid state by heat after the evaporation of the solvent, the filler retracting between the grains to an extent such that surface parts of the grains become free of the filler, after which the tiller is hardened.

Hardening may be obtained by simple solidification during cooling or, when thermohardening materials are employed, by thermal treatment.

If the filler consists of a thermohardening substance and the constituents of the filler are in the liquid state, the solvent is preferably evaporated at a temperature at which these constituents hardly start reacting with each other or at which these reactions occur only so slowly that after the evaporation of the solvent the liquid state is maintained, after which the filler is hardened, if necessary, at a higher temperature.

Fillers hardening in the cold state may also be used, provided the liquid state is maintained sufficiently long for enabling the manufacture of the layer of grains and the retraction of the filler between the grains. in all these cases shrinkage, if any, is smaller than in the case in which a solvent is evaporated.

The invention may be carried out by means of small size grains, for example, thicknesses of not more than 200p. The method is particularly suitable for the use of grains of to 50 which provide correspondingly thin layers 1 grain thick.

Since the method according to the invention provides a layer of grains, whose surfaces are free of the filler on the side remote from the support, it is possible to provide the grains on this side directly with a layer of an electrode material, which is in intimate contact with the grains. If desired, the free grain surfaces may be treated afterwards, for example they may be selectively etched, as the case may be prior to the application of an electrode layer. In principle, after the formation of a hardened filler layer between the grains, whose surface parts are left free, a second similar layer of the filler may be applied to a quantity such that in the liquid state the filler also retracts between the grains, leaving surface parts of the grains Ul'lcovered. As a matter of course, the quantity of the filler first applied should then not be chosen too large, while it is furthermore not necessary for the first filler to satisfy alone the requirements of cohesion and rigidity of the final layer. The last-mentioned method may be particularly advantageous with the use of grains consisting of a core of one material and an envelope of a further material. The core and the envelope may consist of different constituents, for example difierent semiconductor materials. The core and the envelope may have the same main constituent but having different properties of conductivity, for example by different doping. Particularly important is, for example, the use of semiconductor grains whose envelope forms a rectifying junction, for example, a Phi-junction with the core. After the formation of the first layer of filler the free surface parts of the grains may be subjected to an etching treatment, so that the envelope is locally removed. After the application of the second layer of filler an electrode layer may be applied to the uncovered surface parts of the grains, which layer cannot produce short circuits with the envelope.

In my aforesaid prior patent applications it has been proposed to form semiconductor electrode systems on the basis of cadmium sulphide, cadmium selenide and cadmium telluride or mixed crystals thereof, particularly photocells in the shape of a layer of one grain thick consisting of cadmium chalcogenide, for example, cadmium sulphide grains in a filler, the layer being provided on either side with electrodes. In the manufacture of such electrode systems by the method according to the invention the use of polyurethane as a filler has proved to be particularly suitable. The quantity of this filler may be chosen so that the level of the filler occupies 30 to 80 percent of the height of the grain, while the grains are embraced by the filler up to above their centers, while surface parts of the grains are free of the filler. In the nonhardened state the filler can be heated to such an extent that at the temperature involved it gradually hardens from the liquid state, said liquid state being maintained for a sufficient time to allow the retraction of the filler between the grains.

Layers of grains manufactured by the method according to the invention may be employed for many purposes, which are not restricted to the uses referred to above. Other possibilities of use are, for example, in temperature-dependent resistors, particularly for use in bolometers, nonlinear resistors, diodes and probably in other not yet mentioned semiconductor electrode systems.

it is furthermore known in xerography to obtain an electric charge image corresponding to a desired pattern or picture and developed that is to say rendered visible by means of a colored powder consisting of electrically charged particles on the surface of a photoconductive layer or on an insulating layer being in contact herewith by the projection of the desired pattern or picture onto the photoconductive layer. The latter is then advantageously formed by a layer of one grain thick of the kind set forth. On the one hand the flexibility of such a layer one grain thick facilitates the establishment of a good contact with an insulating layer on which the electric charge image is formed or to which it has to be transmitted and on the other hand the low transverse conduction of such a layer 1 grain thick is conducive to the definition. With this use it is also advantageous that on one side of the layer the surface parts of the grains are free of the tiller. In this case the grains need not be provided on said side with an electrode.

The invention will be described more fully with reference to the accompanying drawing, in which FIGS. ll'and 2 are vertical sectional views of successive stages of the method according to the invention.

FlGS. 3 to '7 are vertical sectional views of successive stages of the manufacture of an electrode system comprising a layer l grain thick by a further method according to the invention and FIGS. to it are vertical sectional views of successive stages of a still further method according to the invention.

The method starts by a layer of grains 3, which are stuck by means of an adhesive layer 2 to the surface of a support i (see FIG. ll). For the manufacture of an electrode system the grains preferably consist of a medium active in such an electrode system, for example of the type described above. The support may consist of an electrically conductive material, for example, a metal foil, there being then used a conductive paste for the adhesive layer, for example, silver paste. The grains may also be applied to the support by means of a soluble adhesive layer in order to remove the layer of grains afterwards, when the filler is applied and in order to obtain free grain surfaces on the side facing the support, as is proposed in my copending US. Pat. application Ser. No. 569,248. The support with the grains adhering thereto is then dipped in molten paraffin. The liquid paraffin adheres with difficulty to grain material suitable as media for electrode systems, for example ceramic materials and semiconductor materials. The liquid paraffin contracts in the space between the grains, while it flows back from surface parts 6 of the grains located on the side of the layer remote from the support (see FIG. 2). The melt recontracted between the grains assumes the form of a convex meniscus 7. By cooling the molten paraffin is caused to solidify so that the support it has a layer of grains 3 embedded in a paraffin filler 5. Since on the side of the resultant layer remote from the support the surface parts 6 of the grains are free of the filler, an electrode can then be arranged on said side, which electrode establishes a good contact with the grains.

In the following example described with reference to FIGS. 3 to 7 a filler is used, which can contract in the liquid state between the grains, but which adheres to the grains surface so that it forms a concave meniscus, whereas it leaves surface parts of the grains free on the side remote from the support.

The support is formed by a flat glass plate 11 to which by means of an adhesive layer of gelatin l2 photocoriductive grains 33 of a diameter of 30 to 40 [.t of cadmium sulphide are adhered (see EH13).

For applying the filler a solution of not yet hardened polyurethane materials is employed. In the present case the commercially available Desmopheen 1200 and Desmodur L are used, which have to be mixed with each other for producing polyurethane. Ethylacetate is then added, which reduces the viscosity. The quantities used of 45 g. of Desmopheen 1200, 45 g. of ethylacetate and 65 g. of Desmodur L" are mixed and the resultant mixture is caused to flow across the surface of the support with the adhering cadmium sulphide grains until a liquid film 14 is formed throughout the graincovered surface. After 21 I5 minutes exposure of the support to the air at room temperature the ethylacetate has evaporated for the major part, after which the assembly is heated for 5 hours at 75 C. The filler has then recontracted in the liquid state between the grains so that on the side remote from the support surface parts 16 of the grains 13 are free of the filler see FIG. 4). The liquid tiller thus retracted between the grains has, however, still such a force of adhesion to the grain surface that it forms a concave meniscus, the level between the grains lying considerably lower than the heads of the grains.

During the thermal treatment the polyrethane gradually hardens by polycondensation of the raw materials used, so that the filler becomes solid. The assembly is then cooled.

Then an electrode layer, for example a transparent electrode can be applied to the grains. Such an electrode layer should, in general, be very thin in order to be sufficiently pervious to the radiation; but this involves a high lateral resistance.

in this example a method is used, which is described in a copending US. ?at. application, Ser. No. 630,000, now abandoned, tiled Apr. 1 l, 1967, in which a thicker, good conducting layer, which is not or only little pervious to radiation, is obtained, which is at least mainly confined to the surface of the filler between the grains, while surface parts of the grains on the side remote from the support are not covered by this good conducting layer. For this purpose, if necessary subsequent to an appropriate preliminary treatment of the free surface parts, for example by means of a glow discharge, such a fairly thick electrode layer 18, for example of indium is first applied to the whole free surface, for example by vapor deposition (see FIG. 5).

Since in the present case, in which the polyurethane filler layer is obtained by hardening a liquid forming a concave meniscus, the grains project considerably above the filler between the grains, the special grinding method described in said copending Application permits of grinding off the projecting parts of the metal layer 18, located on the grains, whereas the lower parts of the layer 18 are maintained (see FlG. s it is then possible to use a grinding material, the grains of which are so large with respect to the distances between the adjacent grains i3 that the grinding material can not penetrate to the deeper parts of the layer 18. By a further grinding method described in said application an abrasive applied to a support may be employed, the grains of which have a diameter which is considerably smaller than the average diameter of the grains of the layer. Also in this manner deeper parts of the layer 18 are protected against the action of the abrasive. The good conducting layer it; is then removed from the surface parts 19 of the grains without, however, the contact between the layer and the grains l3 being interrupted. A thin, radiation-pervious electrode may then be deposited by evaporation on the surface, if desired, the free surface parts 19 of the grains may be rendered better conductive, for example by means of a glow discharge. In this manner the side of the layer of grains remote from the support is provided with an electrode which allows irradiation of the photoconductive grains and which has yet a low lateral resistance. In order to enhance the rigidity of the layer of the layer of grains, a layer of radiation-pervious, plastic substance, for example polyurethane may be applied to the side of the layer of grains remote from the support (said layer is not shown; it may have a thickness of 50 ,u).

By dissolving the adhesive layer 12, for example in the case of a gelatine layer by means of hot water, the layer of grains can "then be removed from the support ll, while, if necessary, residual material of the adhesive layer can be washed away.

The surface parts 20 of the grains initially countersunk in the adhesive layer are free of the filler, so that these surface parts can be provided with a second electrode, for example by the vapor deposition of an indium layer 21 (see FIG. 7). The electrode system obtained may serve as a photoconducting cell.

The material of the metal layer 18 may be copper instead of iridium and a thin, transparent copper layer may be applied by vapor deposition to the free surface parts 19, in which case a photo-voltaic cell is obtained, which may be used as a solar battery.

One form of the method according to the invention will now be described with reference to FIGS. 8 to 11 for the manufacture of electrode systems having a layer of 1 grain thick, the grains being formed by a core and an envelope of materials having different properties. The materials of the core and the envelope may be quite different; for example the core may consist of semiconductor material, and the envelope may consist of a metal or another semiconductor material; the core and the envelope may, however, also consist of the same semiconductor material but of different conductivity types. These grains are stuck to the surface of a support 41 by means of an adhesive layer 42 (see FIG. 8). The grains consist of a core 43 and an envelope ll of materials having different electric properties. in the manner described above the support with the grains is dipped in a solution of the filler, which solution is, however, more diluted. The solvent is evaporated and the filler is hardened by heating. In the liquid state preceding the hardening process the tiller retracts between the grains, as is indicated in FIG. 9 by 45. The grains are then subjected to an etching treatment, so that on the side remote from the support the material of the envelope is removed and the core l3 obtains a free surface 46. On the side facing the support the material of the envelope is maintained. lf electrode material were applied to the surface parts 46, for example by vapor deposition, this material could, as shown in the case of FIG. 10, still establish a contact with the edge of the material Ml of the envelope, so that the core 43 and the envelope 44 would be short circuited. in order to avoid such a short-circuit the support with the grains is again dipped in a diluted solution of the filler. In principle, the same filler or a different may be chosen as before. After the evaporation of the solvent the new filler is hardened, for example, by heating While in the liquid state preceding the hardening process the filler retracts also from the upper sides of the grains, but to a lesser extent than the tiller 35. In this manner also the edges of the envelopes 44 are covered by the new filler 47 (see FIG. 11). Since, however, surface parts as of the grains are free of the fillers, an electrode layer can be applied similarly to the method described above to establish a satisfactory contact with the core material 43, whereas it is insulated from the envelope material 54. The electrode for the envelope surface may be formed by a conducting adhesive layer 42 or after the solution of the adhesive layer 512 the electrode may be applied, for example by vapor deposition.

What is claimed is:

l. in a method of manufacturing an electrical device comprising a granular layer of electrically active grains bound together by an electrically insulating filler and having electrode means therefor with at least one of the electrodes having electrically conductive portions on exposed surface parts of the grains, the improvement comprising the steps of applying to a layer of said grains on a support an electrically insulating filler substance in the liquid state, contracting the filler between the grains whereby its level between the grains lies below the grain peaks exposing the latter, and thereafter hardening the filler with the grain peaks exposed.

2. A method as set forth in claim 1 wherein, after hardening of the tiller, an electrically conductive coating is applied to the exposed grain peaks.

3. A method as set forth in claim ll wherein the filler comprises a resin hardenable by polymerization or polycondensation in a liquid solution with a volatile solvent, said filler being hardened after evaporation of the solvent.

4. A method as set forth in claim 3 wherein the filler com- 6. A method as set forth in claim 1 wherein the grain prises polyurethane. thickness is not more than 200 microns.

5. A method as set forth in claim 43 wherein the grains com- A method as forth in claim 6 wherein the grain prise semiconductive material of a chalcogenide of cadmium, thlckness 10*50 zinc or lead. 

2. A method as set forth in claim 1 wherein, after hardening of the filler, an electrically conductive coating is applied to the exposed grain peaks.
 3. A method as set forth in claim 1 wherein the filler comprises a resin hardenable by polymerization or polycondensation in a liquid solution with a volatile solvent, said filler being hardened after evaporation of the solvent.
 4. A method as set forth in claim 3 wherein the filler comprises polyurethane.
 5. A method as set forth in claim 4 wherein the grains comprise semiconductive material of a chalcogenide of cadmium, zinc or lead.
 6. A method as set forth in claim 1 wherein the grain thickness is not more than 200 microns.
 7. A method as set forth in claim 6 wherein the grain thickness is 10-50 microns. 